1️⃣ In a medium saucepan, stir together the basil sprigs, 4 cups water, sugar and lemon peel. Bring a slow boil and continue cooking, stirring, until the sugar is dissolved. Let sit at room temperature for around an hour, then refrigerate for at least another hour (I like to leave it overnight). Strain the mixture through a fine mesh sieve into a mixing bowl, pushing down on the solids to squeeze out as much of the liquid as possible.
2️⃣ Mix together the basil mixture with the remaining 2 to 3 cups water (to taste) and the lemon juice. Chill for at least an hour. Serve over ice, garnished with a basil sprig.